240 X OF SENSATION, AND THE ORGANS OF THE SENSES. 



odours, is by no means constant amongst different individuals. Many of the 

 lower animals pass their whole lives in the midst of odours, which are to Man 

 (in his civilized condition at least) in the highest degree revolting ; and will 

 even refuse to touch food, until it is far advanced in putridity. It more fre- 

 quently happens in regard to odours and savours, than with respect to other 

 sensory impressions, that habit makes that agreeable, and even strongly re- 

 lished, which was at first avoided ; the taste of the epicure for game that has 

 acquired thefumet, for olives, for assafcetida, &c., are instances of this. As 

 to the length of time, during which impressions made upon the organ of smell 

 remain u^>on it, no certain knowledge can be obtained. It is difficult to say 

 that the effluvia have been completely removed from the nasal passages ; since 

 it is not improbable that the odorous particles (supposing such to exist) are 

 absorbed or dissolved by the mucous secretion ; it is probably in this manner 

 that we may account for the fact, well known to every medical man, that the 

 cadaverous odour is frequently experienced for days after a post-mortem ex- 

 amination.* 



V. Sense of Vision. 



326. The objects of this sense are bodies, which are either in themselves 

 luminous, or which become so by reflecting the light that proceeds from others. 

 Whether their light is transmitted by the actual emission of rays, or by the 

 propagation of undulations analogous to those of sound, is a question at present 

 keenly debated amongst Natural Philosophers ; but it is of little consequence 

 to the Physiologist, which is the true solution ; since it is only with the laws 

 which actually regulate the transmission of light, that he is concerned. These 

 laws it may be desirable here briefly to recapitulate. 



327. Every point of a luminous body sends off a number of rays, which 

 diverge in every directidn, so as to form a cone, of which the luminous point is 

 the apex. So long as these rays pass through a medium of the same density, 

 they proceed in straight lines ; but, if they enter a medium of different density, 

 they are refracted or bent, towards the perpendicular to the surface at the 

 point at which they enter, if they pass from a rarer into a denser medium, 

 and from the perpendicular, when they pass from a denser medium into a 

 rarer. It is easily shown to be a result of this law, that, when parallel rays 

 passing through air fall upon a convex surface of glass, they will be made to 

 converge ; so as to meet at the opposite extremity of the diameter of the 

 circle, of which the curve forms part. If, instead of continuing in the glass, 

 they pass out again, through a second convex surface, of which the direction 

 is the reverse of the first, they will be made to converge still more, so as to 

 meet in the centre of curvature. Rays which are not parallel, but which are 

 diverging from a focus, are likewise made to converge to a point or focus ; 

 but this point will be more distant from the lens, in proportion as the object 

 is nearer to it, and the angle of divergence consequently greater. The rays 

 diverging from every point of a luminous object are thus brought to a corre- 

 sponding focus ; and the places of all these foci hold exactly the same relation 

 to each other, with that of the points from which the rays diverged ; so that 

 a perfect image of the object is formed upon a screen held in the focus to the 

 lens. This image, however, will be inverted ; and its size, in proportion to that 

 of the object, will depend upon their respective distances from the lens. If 

 their distances be the same, their size will also be the same ; if the object be 

 distant, and the image near, the latter will be much the smaller ; and" vice 

 versa. 



* This may partly be attributed also to the effluvia adhering to the dress. It has been 

 remarked that dark cloths retain these more strongly than light. 



